Gender differences in regard to aggressive behaviour were investigated in a series of studies of schoolchildren of different age cohorts: 8-year-olds (N = 85), 11-year-olds (N = 167), and 15-year-olds (N = 127). Different types of aggressive behaviour were measured with peer nomination techniques, supported by self-ratings. The social structure of the peer groups were also studied. The results of the 11-year-old cohort were previously presented by Lagerspetz et al. [1988; Aggressive Behavior 14:403–414], but they are compared here with the other age groups. The principal finding was that girls of the two older cohorts overall make greater use of indirect means of aggression, whereas boys tend to employ direct means. Previously, the main difference between the genders has been thought to be that boys use physical aggressive strategies, while girls prefer verbal ones. Our studies suggest that the differentiation between direct and indirect strategies of aggression presents a more exact picture. Indirect aggressive strategies were not yet fully developed among the 8-year-old girls, but they were already prominent among the 11-year-old girls. Aggressive behaviour was assessed overall by the children themselves to be the highest in this age group.